An antimaximum principle for periodic solutions of a forced oscillator

Consider the equation of the linear oscillator u '' + u = h(theta), where the forcing term h : R -> R is 2 pi-periodic and positive. We show that the existence of a periodic solution implies the existence of a positive solution. To this aim we establish connections between this problem and some separation questions of convex analysis.Paris Observatory gran

How many Keplerian arcs are there between two points of spacetime?

We consider the Keplerian arcs around a fixed Newtonian center joining two prescribed distinct positions in a prescribed flight time. We prove that, putting aside the "opposition case" where infinitely many planes of motion are possible, there are at most two such arcs of each "type". There is a bilinear quantity that we call b which is in all the cases a good parameter for the Keplerian arcs joining two distinct positions. The flight time satisfies a "variational" differential equation in b, and is a convex function of b.Comment: 16 pages, 3 figure

Vitamin D, a modulator of musculoskeletal health in chronic kidney disease

The spectrum of activity of vitamin D goes beyond calcium and bone homeostasis, and growing evidence suggests that vitamin D contributes to maintain musculoskeletal health in healthy subjects as well as in patients with chronic kidney disease (CKD), who display the combination of bone metabolism disorder, muscle wasting, and weakness. Here, we review how vitamin D represents a pathway in which bone and muscle may interact. In vitro studies have confirmed that the vitamin D receptor is present on muscle, describing the mechanisms whereby vitamin D directly affects skeletal muscle. These include genomic and non‐genomic (rapid) effects, regulating cellular differentiation and proliferation. Observational studies have shown that circulating 25‐hydroxyvitamin D levels correlate with the clinical symptoms and muscle morphological changes observed in CKD patients. Vitamin D deficiency has been linked to low bone formation rate and bone mineral density, with an increased risk of skeletal fractures. The impact of low vitamin D status on skeletal muscle may also affect muscle metabolic pathways, including its sensitivity to insulin. Although some interventional studies have shown that vitamin D may improve physical performance and protect against the development of histological and radiological signs of hyperparathyroidism, evidence is still insufficient to draw definitive conclusions

Forest Ecosystem Services: An Analysis of Worldwide Research

The relevance of forests to sustain human well-being and the serious threats they face have led to a notable increase of research works on forest ecosystem services during the last few years. This paper analyses the worldwide research dynamics on forest ecosystem services in the period from 1998 to 2017. A bibliometric analysis of 4284 articles was conducted. The results showed that the number of published research articles has especially increased during the last five years. In total, 68.63% of the articles were published in this period. This research line experiences a growing trend superior to the general publishing trend on forest research. In spite of this increase, its relative significance within the forest research is still limited. The most productive subject areas corresponded to Environmental Science, Agricultural and Biological Sciences and Social Sciences Economic topics are understudied. The scientific production is published in a wide range of journals. The three first publishing countries are United States, China and the United Kingdom. The most productive authors are attached to diverse research centres and their contributions are relatively recent. A high level of international cooperation has been observed between countries, institutions and authors. The findings of this study are useful for researchers since they give them an overview of the worldwide research trends on forest ecosystem services

Efecto de un trabajo de aprendizaje del ciclo estiramiento-acortamiento sobre la capacidad de salto en voleibol

El propósito de esta investigación es conocer el efecto sobre la capacidad de salto (tiempo de ejecución y altura alcanzada por el centro de gravedad) de un trabajo de aprendizaje de la utilización del ciclo estiramiento - acortamiento de la musculatura flexora - extensora del pie a través de ejercicios de técnica de carrera realizados de forma integrada dentro del entrenamiento. Se realizó un estudio intergrupo con 12 jugadores de voleibol de nivel nacional divididos aleatoriamente en dos grupos (experimental y control). Todos los jugadores realizaron el mismo entrenamiento durante el periodo de estudio (primer macrociclo competitivo). El grupo experimental desarrolló de forma adicional en la parte final del calentamiento el trabajo de aprendizaje tres días por semana durante 14 semanas. Antes y después del trabajo de aprendizaje se realizó una batería de control, en donde se registró la altura alcanzada y el tiempo de impulso, en fase excéntrica y concéntrica. El grupo experimental consiguió una mejora significativa (disminución) a nivel de tiempo de la contracción excéntrica y del tiempo total de impulso en la prueba de bloqueo con desplazamiento lateral sin cruce. Respecto a la altura alcanzada no se observa diferencia entre ambos grupos. El efecto de un trabajo de aprendizaje produce una ganancia temporal en la acción de bloqueo con desplazamiento lateral sin cruce en voleibol aplicando los mismos niveles de fuerza (altura alcanzada)

On the dynamics of a quadratic scalar field potential

We review the attractor properties of the simplest chaotic model of inflation, in which a minimally coupled scalar field is endowed with a quadratic scalar potential. The equations of motion in a flat Friedmann-Robertson-Walker universe are written as an autonomous system of equations, and the solutions of physical interest appear as critical points. This new formalism is then applied to the study of inflation dynamics, in which we can go beyond the known slow-roll formalism of inflation.Comment: 14 pages, 6 eps figures, matches version to appear in IJMP